Haematological responses in the European eel, Anguilla anguilla L., to sublethal infestation by Anguillicola crassus in a thermal effluent of the Swedish Baltic

. Haematological variations were studied in a European eel population, Anguilla anguilla L., naturally infested by the sanguivorous nematode Anguillicola crassus. This occurred off the Swedish Baltic coast in an area receiving heated cooling water from a nuclear power station. The erythrocyte count, haematocrit, leucocrit, haemoglobin concentration, mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentrations, the various cell types in peripheral blood, serum iron concentration, the amount of total serum protein, and scrum protein fractions α1, β1, α2, β2 and γ were all measured. Each variable was correlated to: (1) the intensity of infestation, i.e. number of parasites per host; and (2) a parasitization index, i.e. the weight of parasites per somatic weight of the host. Most variables showed no or only minor reactions to the infestation. However, a marked increase in the γ‐fraction of scrum proteins, reduced lymphocyte numbers and increased granulocyte numbers are indicative of a humoral and cellular immune response.